Chemical mechanical polishing (CMP) apparatus is widely known as a polishing apparatus for polishing a surface of a substrate, such as a wafer. This polishing apparatus polishes the substrate by holding the substrate with a top ring and pressing the substrate against a polishing pad on a polishing table while moving the polishing table and the top ring relative to each other. During polishing of the substrate, the polishing pad is supplied with a polishing liquid (or slurry) from a polishing liquid supply nozzle, so that the surface of the substrate is polished by a chemical action of the polishing liquid and a mechanical action of abrasive grains that are contained in the polishing liquid.
As the substrate is continuously polished with use of the polishing pad, minute irregularities that constitute the surface of the polishing pad are flattened, thus causing a decrease in a polishing rate. It has been customary to dress (or condition) the surface of the polishing pad with a dresser (or a pad conditioner) having a number of abrasive grains, such as diamond particles, electrodeposited thereon in order to recreate the minute irregularities of the polishing pad surface. During dressing of the polishing pad, a dressing liquid, such as pure water, is supplied onto the polishing pad.
FIG. 1 is a side view of a dresser 2 when dressing a polishing pad 3 on a polishing table 1. FIG. 2 is a plan view showing relative movement between the polishing table 1 and the dresser 2. The polishing pad 3 is mounted to the polishing table 1 and has an upper surface serving as a polishing surface 3a. In FIG. 2, dot-and-dash lines indicate oscillation of the dresser 2, and two-dot-and-dash lines indicate a plurality of regions defined on the polishing surface 3a of the polishing pad 3. As shown in FIG. 1, the dresser 2 is supported by a dresser arm 6. The dresser arm 6 is configured to pivot on a dresser pivot shaft 5. When the dresser arm 6 pivots, the dresser 2 oscillates on the polishing surface 3a in substantially a radial direction of the polishing surface 3a. The dresser 2 has a lower surface serving as a dressing surface constituted by a number of abrasive grains, such as diamond particles. While oscillating on the polishing surface 3a, the dresser 2 rotates on the polishing surface 3a to scrape away the polishing pad 3 slightly, thereby dressing the polishing surface 3a. 
The dresser 2 is coupled to a dresser shaft 4 via a spherical bearing, not shown in the drawing. The dresser shaft 4 is configured to apply a load to the center of the dresser 2 to press the dressing surface of the dress 2 against the polishing surface 3a of the polishing pad 3. Therefore, when the dressing surface of the dresser 2 is placed in sliding contact with the polishing surface 3a by the rotations of the polishing table 1 and the dresser 2, the dresser 2 is tilted with respect to the polishing surface 3a of the polishing pad 3 due to frictional resistance that is generated between the polishing surface 3a and the dresser 2. If the tilted dresser 2 dresses the polishing surface 3a of the polishing pad 3, a peripheral portion of the dresser 2 is worn more quickly than the central portion of the dresser 2. As a result, the dresser 2 has to be replaced with a new one even before the central portion of the dresser 2 is worn. Therefore, a replacement frequency of the dresser 2 increases.
In the example shown in FIG. 2, a plurality of concentric annular or circular regions R1 through R5 are defined on the polishing surface 3a of the polishing pad 3. For example, the region R5 is an outermost circumferential region of the polishing pad 3, and the region R1 is a central region of the polishing pad 3. Since these regions R1 through R5 have different radii, a velocity of the polishing pad 3 in its circumferential direction varies from region to region. When the dresser 2 moves across the multiple regions R1 through R5 of the polishing pad 3, the dresser 2 is tilted in various ways due to the different velocities in the respective regions. As a consequence, the dresser 2 cannot uniformly dress the polishing surface 3a in its entirety, failing to make the polishing surface 3a flat.